The present invention relates to polymer stabilized liquid crystals (PSLC's), methods of manufacturing such PSLC's, and fabrication of flexible PSLC films. The present invention also relates to methods of making flexible polymer-free liquid crystal films.
One known method of creating stabilized liquid crystals is Polymer Dispersed Liquid Crystal (PDLC) technology. PDLC technology typically consists of dispersions of liquid crystals microdroplets in a polymer matrix at a liquid crystal concentration range of 30 to 80 weight percent. The liquid crystal is the discontinuous phase and the matrix is the continuous phase. Among the advantages of PDLC films over conventional liquid crystal dispersions (LCD's) are the ease of manufacturing on large roll-to-roll plastic supports and in the manufacture of switchable windows and displays. However, PDLC technology suffers from a number of inherent disadvantages, among which the index mismatching (haze), high voltages required, lack of resin stability, color, and lack of reverse-mode capability (i.e., off-state transparency/on-state opacity) are major problems.
In recent years, another complementary technology to both PDLC and LCD, referred to as Polymer Stabilized Cholesteric Texture (PSCT) has been developed. PSCT is based on colored or non-colored displays prepared by gelification of a mixture of about 5 weight percent ultraviolet radiation-curable prepolymer and greater than 95 weight percent cholesteric liquid crystal. After curing the display consists of a continuous liquid crystal phase stabilized (gelified) by the polymer network. Due to the high concentration of liquid crystal in PSCT, the gel display has to be prepared between rigid sealed glass supports; this requirement is the main disadvantage of this technology for large shutters and displays. In spite of this disadvantage, PSCT has other promising advantages with respect to both LCD and PDLC that can be summarized as follows: (1) haze-free normal-mode and reverse-mode shutters (compared to PDLC); (2) simplicity of fabrication by eliminating the polarizers and dyes (compared to LCD); (3) low voltage requirement (compared to PDLC); and (4) bistability (compared to LCD and PDLC).
Regardless of the method of preparation of the starting materials and processing method, the cholesteric texture in PSCT devices consists of the three basic structures: planar texture, focal conic texture, and homeotropic texture. to create a normal mode PSCT device, a homogeneous mixture of prepolymer (2 weight percent) and cholesteric liquid crystal (98 weight percent) may be filled inside a 5-micron thick sealed glass "bottle" and cured under a magnetic field. After curing and removal of the field (Off-state), the texture of the cholesteric phase (having its pitch in the IR region) becomes polydomains with focal conic texture, and due to intense light scattering, the film shows intense opacity. Upon the application of the field (On-state), the cholesteric pitch unwinds and, by changing to a homoeotropic nematic texture, the film become transparent. As the concentration of polymer gel is low, there will be no index mismatching and the shutter in the On-state is transparent in all viewing directions (haze-free).
To create a reverse mode PSCT film, a homogeneous mixture of an ultraviolet radiation-curable prepolymer (2 weight percent) and cholesteric liquid crystal (98 weight percent) may be filled inside a 5 to 10 micron thick sealed glass bottle and the film cured without any magnetic field. The internal metallized surfaces of the glass supports are coated with polyimide and buffed unidirectionally in order to induce a parallel (planar) alignment of the liquid crystal molecules. After curing, the planar texture of the cholesteric phase (having its pitch in the IR region) shows complete transparency in the field-off state. In the On-state, the field induces a random disorientation of cholesteric pitch axis, causing the establishment of focal conic texture where the film becomes opaque. Due to low concentration of polymer gel, there will be no index mismatching and the shutter in the Off-state is transparent in all viewing directions (haze-free).
In the bistable reflective mode, if the liquid crystal material has a cholesteric pitch in the visible range of light, the display may be driven from a colored reflective mode to a scattering mode to a transparent mode.
The instant invention relates to PSLC films containing microparticles which act as microspacers as well as PSLC films having replicated structures. The present invention also relates to dual-mode and haze-free liquid crystal film technology that has dispersions of mesogenic polymer network in a continuous cholesteric or nematic liquid crystal matrix. The current invention also relates to methods for preparing PSLC on large flexible films by use of binder fixed spacers and by fixed replicated structures made of binder. The invention also relates to methods of making flexible liquid crystal devices containing polymer-free liquid crystal.